A primary problem associated with electrophotographic device reproduction of color is reproducibility. Users of such devices expect stability in color rendition within a page, from page to page, and from job to job. One method of maintaining color reproducibility with process colorants is to achieve stable gray balance.
Imaging processes wherein a developed image is first transferred to an intermediate transfer support and subsequently transferred from the intermediate transfer support to a support are known. For example, U.S. Pat. No. A 3,862,848 (Marley), discloses an electrostatic method for the reproduction of printed matter in which an electrostatic latent image is developed by the attraction of electroscopic marking particles thereto and is then transferred to a first receptor surface by the simultaneous application of contact and a directional electrostatic field of a polarity to urge the marking particles to the receptor surface, with the image then being transferred from the first receptor surface to a second receptor surface by the simultaneous application of contact and a directional electrostatic field of opposite polarity to urge the marking particles to the second receptor surface.
A primary problem associated with such intermediate transfer arrangements is that upon the electrostatic transfer of toner from a first support to a second, i.e., from photoreceptor to intermediate, photoreceptor to paper, or intermediate to paper, some residual amount of toner is always left on the first support. In color applications, having three or four layers of toner, this suggests that the layer of toner on the first support farthest from the second support will have some residual amount of toner left on the first surface, i.e., transfer efficiency of this layer is less than 100%. If the amount of this residual is known prior to imaging and is stable within a page, from page to page, and from job to job, then a precompensation process can be put into place to maintain gray balance. However, if there is instability in the residual amount, or if it is a function of image content or location, then the best that may be done is to alleviate the impact of instability.
U.S. Pat. No. A 3,957,367 to Goel, discloses an example color electrostatographic printing machine in which successive single color powder images are transferred, in superimposed registration with one another, to an intermediary. The multi layered powder image is fused on the intermediary and transferred therefrom in a single step to a sheet of support material, forming a copy of the original document.
U.S. Pat. No. A 3,893,761 to Buchan et al., discloses an apparatus for transferring non fused xerographic toner images from a first support material, such as a photoconductive insulating surface, to a second support material, such as paper, and fusing the toner images to the second support material. Toner images are transferred from the first support material to the intermediate transfer member by any conventional method, preferably pressure transfer. The toner image is then heated on the intermediate transfer member to at least its melting point temperature, with heating preferably being selective. After the toner is heated, the second support material is brought into pressure contact with the hot toner whereby the toner is transferred and fused to the second support material.
U.S. Pat. No. A 4,682,880 (Fujii et al.), discloses a process wherein an electrostatic latent image is formed on a rotatable latent image bearing member and is developed with a developer into a first visualized image. The first visualized image is transferred by pressure to a rotatable visualized image bearing member. The steps are repeated with different color developers to form subsequent visualized images on the same visualized image bearing member to constitute a multi color image which corresponds to one final image to be recorded. The latent image bearing member and the visualized image bearing member form a nip therebetween through which a recording material is passed so that the multi color image is transferred all at once to a recording material.
"Color Xerography With Intermediate Transfer," J. R. Davidson, Xerox Disclosure Journal, Volume 1, Number 7, page 29 (July 1976), the disclosure of which is incorporated herein by reference, discloses a xerographic development apparatus for producing color images. Registration of the component colors is improved by the use of a dimensionally stable intermediate transfer member. Component colors such as cyan, yellow, magenta, and black are synchronously developed onto xerographic drums and transferred in registration onto the dimensionally stable intermediate transfer member. The composite color image is then transferred to a receiving surface such as paper. The intermediate transfer member is held in registration at the transfer station for transferring images from the xerographic drums to the member by a hole-and-sprocket arrangement, wherein sprockets on the edges of the drums engage holes in the edge of the intermediate transfer member.
EP-A Publication No. 0 453 762 (A2) discloses an imaging apparatus and process wherein an electrostatic latent image is formed on an imaging member and developed with a toner, followed by transfer of the developed image to an intermediate transfer element and subsequent transfer with very high transfer efficiency of the developed image from the intermediate transfer element to a permanent support.
Intermediate transfer elements employed in imaging apparatuses in which a developed image is first transferred from the imaging member to the intermediate and then transferred from the intermediate to a final support should exhibit both good transfer of toner material from the imaging member to the intermediate and very good transfer of toner material from the intermediate to the support. Very good transfer occurs when most or all of the toner material comprising the image is transferred and little residual toner remains on the surface from which the image was transferred. In order for transfer to be very good, it also has to have a very low variability in the residual toner from job to job, image to image, point to point. Very good transfer is particularly important when the imaging process entails generating full color images by sequentially generating and developing images in each primary color in succession, and superimposing the primary color images onto each other on the intermediate. Undesirable shifting or color deterioration in the final colors obtained can occur when there is variability in the primary color image transfer from the intermediate to the support (paper).
Although known methods and materials are suitable for their intended purposes, a need remains for imaging apparatuses and methods employing intermediate transfer elements with high transfer efficiency to a final support. In addition, there is a need for imaging apparatuses and methods employing intermediate transfer elements that enable generation of full color images with high color image quality and stable color reproduction. Stable color reproduction is associated with maintenance of gray balance during transfer.
One of the characteristics of intermediate transfer (i.e., transfer from a photoreceptor to an intermediate and subsequent retransfer to a final support) is that the top layer during the first transfer becomes the bottom layer during the second transfer, It has been observed that the bottom layer (i.e., the layer closest to the first support and furthest from the second support) generally tends to leave the greatest and most variable amount of residual toner on the support. Accordingly, the contribution to the appearance of an image by this toner is variably affected, especially if the image consists of at least three toner layers.
U.S. Pat. No. A 4,833,503 to Snelling U.S. Pat. No. A 4,819,028 to Abe and U.S. Pat. No. A 4,660,059 to O'Brien, all disclose methods of forming and developing multiple layers of toner on a surface for subsequent simultaneous transfer to a final support or receiving member. While, strictly speaking, there is no transfer from an intermediate to a final support in the described arrangements, these arrangements and the previously described tandem engine arrangements require a simultaneous transfer of multiple layers of toner, from a first support to a second which tends to leave a residual primarily composed of toner from the layer closest to the first support.